The invention relates to optical multiplexers and demultiplexers and is particularly applicable to optical filters.
Optical filters may comprise several input and output waveguides and a dispersive element such as a diffraction grating located between the input and output waveguides. Light consisting of multiple wavelengths may be input so as to interact with the dispersive element resulting in various frequency components coupling with different output waveguides. The arrangement may include focussing elements such as lenses or mirrors on either side of the dispersive element. The dispersive element which separates the light beam with respect to frequency may be an array of waveguides or etched mirror elements. Such optical filters can be used as multiplexers and demultiplexers in optical systems for combining or separating multiple frequencies of light in wavelength division multiplexed fibre optic communication systems. Such a prior art system is shown in FIG. 1 of the accompanying drawings.
The passband width and shape for each of the dispersed frequencies is important in multiplexer and demultiplexer components. To prevent significant light intensity fluctuations over the passband width of each of the output waveguides, a flattened profile within the passband may be desirable while maintaining a low level of cross talk between the different frequency channels. Wider separation between the output waveguides results in lower cross talk at the expense of narrower passband widths of each channel due to the higher spatial dispersion required. One prior art method of obtaining a flatter profile is the use of a multimode interference coupler of the type shown in the prior art arrangement of FIG. 2. In that case light of multiple wavelengths is input to an input waveguide 11, the light having a field profile of the type shown at 12. The input waveguide 11 may be a single mode waveguide or the profile 12 may represent that of the fundamental mode if the input waveguide is not single moded. In the coupler 13 multimode interference occurs causing an output field profile as shown at 14. When used in the general arrangement of FIG. 1, this results in a flatter field profile for each of the dispersed frequencies received in the output channels thereby reducing fluctuations in light intensity received by each output channel due to tolerance in frequency controls.
It is an object of the present invention to provide improved apparatus and methods for achieving broader and/or flatter passbands which may be used in optical multiplexers, demultiplexers and optical filters.
The invention provides an optical filter comprising at least one optical input for inputting multiple optical wavelengths, a wavelength dispersive device arranged to receive light from the optical input, and a plurality of optical outputs for receiving respective wavelength bands of light from the wavelength dispersive device, at least one of the optical outputs or optical input including at least two outer waveguides on either side of an inner waveguide, the outer and inner waveguides forming a group of coupled waveguides side by side, with the inner waveguide between the two outer waveguides such that evanescent coupling occurs between the outer and inner waveguides.
Preferably the outer waveguides are curved outwardly away from the inner waveguide at an end remote from the dispersive device thereby reducing the loss of the coupler.
Preferably the evanescent waveguide coupler including said inner and outer waveguides is located in said optical input, the inner waveguide being optically coupled to a light source of multiple wavelengths.
Preferably the waveguides of the or each evanescent waveguide coupler have an interaction length and separation to produce overlapping peaks of light intensity transverse to the optical path.
Preferably the outer waveguides are tapered outwardly at the end nearest the wavelength dispersive device thereby to provide a desired overlap of light intensity peaks for light coupled in the coupler.
Preferably said inner waveguide is a single mode waveguide device.
The wavelength dispersive device may include a dispersive grating.
The wavelength dispersive device may include a dispersive waveguide array.
Each optical output may include an inner waveguide coupled to the wavelength dispersive device via at least two outer waveguides on either side of the inner waveguide such that evanescent coupling occurs between the inner and outer waveguides, the two outer waveguides being arranged to receive respective parts of the dispersed output.
Preferably the input forms at least part of an integrated chip device.
The invention includes a method of multiplexing or demultiplexing optical signals by passing light of multiple wavelengths through an optically dispersive device located between input and output waveguides, at least one of the input or output waveguides forming part of an evanescent waveguide coupler comprising a group of coupled waveguides side by side such that optical coupling occurs between outer and inner waveguides of the group.
Preferably light of multiple wavelengths is filtered by a wavelength dispersive device, said method comprising inputting light to the wavelength dispersive device through an optical input and receiving light from the dispersive device through a plurality of spatially separated outputs, the optical input including at least two outer waveguides on either side of an inner waveguide, the outer and inner waveguides forming a group of coupled waveguides side by side, with the inner waveguide between the two outer waveguides such that the evanescent coupling occurs between the inner and outer waveguides.
Preferably each output provides a transmission variation with frequency which is flatter than a Gaussian distribution.
Preferably each output provides a transmission variation with frequency which is broader than a Gaussian distribution.